1. Field of the Invention
The present invention relates to a DC motor stop detector that detects the stopping of a DC motor for use in a CD (compact disk) player, LD (laser disk) player or the like, for example, to drive a disk. This invention also relates to a DC motor brake apparatus that employs the above DC motor stop detector for effectively stopping such a DC motor.
2. Description of the Related Art
With the use of a CD player or LD player, in unloading a disk when playing a music is completed, this process should begin after the disk completely stops rotating.
Stopping the rotation of the disk requires that a brake voltage be supplied to a DC motor which is driving the disk and that the brake voltage be set off when the motor stops rotating. The timing for setting the brake voltage off should be exact, and if it deviates, the forward rotation of the motor may be kept due to the inertia of the motor or the motor may run in the reverse direction. This is likely to damage the disk.
According to a conventional brake apparatus for a DC motor for use in a CD player to drive a compact disk, a motor-rotations detecting disc having a plurality of transparent hole sections and light-shielding sections alternately arranged in the rotational direction is mounted to the rotary shaft of the DC motor. A photosensor is disposed in such a way that part of the periphery of the disc screens the light path of the photosensor. Since the light-shielding sections screen the light path as the disc rotates, the photosensor produces a pulse signal. A rotation detector comprising a counter, etc. counts the number of pulses of the signal per a given time to detect the number of rotations of the motor. There is also a system known which attains such a pulse signal by a combination of a disc having magnetic materials and non-magnetic materials alternately arranged thereon and a magnetic head.
In applying brake, a brake voltage with the opposite polarity to that of a drive voltage is applied to the motor to forcibly weaken the rotational force. When the rotation detector monitoring the rotation of the motor detects that the number of motor's rotations becomes equal to or less than a given value, applying the brake voltage to the motor is stopped. The motor will eventually stop rotating due to its own frictional force.
However, the above conventional DC motor brake apparatus requires a device to detect the rotation of a DC motor, thus increasing the quantity of components. This naturally increases the cost. Since the resolution for detecting the motor's rotations is constant irrespective of the speed of the motor, it would take more time as the motor speed gets slower. This is likely to deviate the time at which generation of the brake voltage should be stopped, thus causing the DC motor to run in the reverse direction.
In short, since the conventional DC motor brake apparatus determines the timing for stopping braking based on the number of rotations of the motor detected, some means or device for detecting the motor's rotations is necessary. In addition, as braking is applied the motor to slow down its speed, it would take more time to detect the number of motor's rotations, which may deviate the timing for stop the braking and cause the motor to run in the reverse direction.